The present invention pertains to a DCxe2x80x94DC converter that supplies a prescribed voltage to a load circuit corresponding to the supplied power source voltage.
A DCxe2x80x94DC converter is usually used in order to convert a DC voltage supplied from a power source to a desired voltage that is different from the power source voltage. The DCxe2x80x94DC converter is composed of switching elements and an inductive element. When the switching elements are turned on/off, current flows in the inductive element, and, as a result, the stored energy is supplied to the load side. By controlling the on/off timing of the switching elements, it is possible to supply the desired voltage that is different from the power source voltage to the load.
FIG. 8 is a diagram illustrating the constitution of an example of a conventional DCxe2x80x94DC converter. The DCxe2x80x94DC converter shown in this figure is composed of switching elements (hereinafter referred to as switches) S1, S2, S3, S4, inductive element L1, and load capacitor Cout. For example, inductive element L1 may be a coil or the like. In the following, it will simply be referred to as inductor L1.
By means of a controller not shown in the figure, switches S1-S4 are controlled to turn on/off. For the DCxe2x80x94DC converter shown in FIG. 8, there are two operating states, state 1 and state 2. These operating states will be explained below.
In state 1, switches S1 and S3 are kept on, and switches S2 and S4 are kept off. In this case, as power source voltage Vin is applied across inductor L1, current flows from the power source voltage supply terminal in the path through switch S1, inductor L1, and switch S3. Thus, energy is stored in inductor L1.
Then, in state 2, switches S1 and S3 are kept off, and switches S2 and S4 are kept on. As a result, the energy stored in inductor L1 in state 1 is released through switch S4 to the load circuit.
By means of the controller, for example, switches S1-S4 are turned on/off at a prescribed timing corresponding to a prescribed clock signal, and said state 1 and state 2 are entered repeatedly. By controlling the time ratio of state 1 and state 2 by means of a clock signal, one can supply a voltage higher or lower than power source voltage Vin to the load circuit. Thus, the DCxe2x80x94DC converter shown in FIG. 8 is also known as up/down converter. Because switches S2 and S4 are only required to have a rectifying effect, switches S2 and S4 may be made up of diodes. However, in this case, electric power losses occur due to the forward voltage drop of the diode. When high efficiency is required, all of switches S1, S2, S3, S4 are all MOSFETs or other transistor elements, and the system is known as synchronized rectifying system.
Said DCxe2x80x94DC converter can be either a boost converter or a buck inverter, which supplies the desired stable voltage to the load circuit. In addition, since the circuit can be formed in a small size, it is now widely used.
However, when the DCxe2x80x94DC converter of the aforementioned synchronized rectifying system is operated at low loads, the inductor draws current from the output side, that is, the current flows backwards in the inductor. This will be explained below with reference to the inductor current IL waveform.
FIG. 9 illustrates the waveforms in an example of the current flowing through a coil. FIG. 9(a) shows the waveform of the clock signal for controlling the on/off timing of switches S1-S4. FIG. 9(b) shows current IL through inductor L1.
In this case, for example, it is assumed that the controller sets the DCxe2x80x94DC converter in state 1 when the clock signal is at the high level, and sets the DCxe2x80x94DC converter in state 2 when the clock signal is at the lower level. Consequently, in state 1, power source voltage Vin applied across inductor L1, so that current IL of inductor L1 rises at a rate of Vin/L. Here, L represents the inductance of inductor L1. As shown in FIG. 9(b), in state 1, current IL1 through inductor L1 rises at a rate of Vin/L.
Then, in state 2, the energy stored in inductor L1 is released to the load circuit. In this case, because output voltage Vout is applied across inductor L1, current IL2 in inductor L1 falls at a rate of Vout/L.
For inductor L1, as current is supplied to the load side in state 2, as shown in FIG. 9(b), by taking average for current IL2 of state 2 in one period of the clock signal, one can determine current Iout supplied by the DCxe2x80x94DC converter to the load circuit.
In the following, the low-load state will be explained. In the low-load state, in state 2, current IL2 output from inductor L1 to the load circuit decreases. When the current drops below zero, reverse current flows from the load circuit to inductor L1. That is, the DCxe2x80x94DC converter sinks current from the load.
FIG. 10 is a waveform illustrating the current through inductor L1 in the low-load state.
As can be seen from this figure, in state 2, the current through inductor L1 falls gradually, and finally becomes negative.
In the low-load state, as reverse current flows through inductor L1, ringing occurs. As a result, an undesirable energy transfer takes place between the input and output, and the conversion efficiency of the DCxe2x80x94DC converter drops, which is problematic.
The purpose of the present invention is to solve the aforementioned problems of the prior art by presenting a DCxe2x80x94DC converter characterized by the fact that in the low-load state, it can prevent reverse current in the inductor, reduce undesirable electric power losses, and increase the conversion efficiency.
In order to realize the aforementioned purpose, the present invention provides a DCxe2x80x94DC converter characterized by the fact that it comprises the following parts: a first switching element connected between one terminal of a voltage source and one terminal of an inductive element; a second switching element connected between the aforementioned terminal of the aforementioned inductive element and reference potential; a third switching element connected between the other terminal of said inductive element and said reference potential; a fourth switching element connected between said other terminal of said inductive element and the output terminal; and a control means which, when said first through fourth switching elements are turned on/off at a prescribed timing, outputs a voltage corresponding to said source voltage to said output terminal, and which turns on said second and third switching elements in the standby mode.
Also, according to the present invention, the following scheme is preferred: said control means has a current detecting means that detects the current through said inductive element, and turns on said second and third switching elements corresponding to the detection result of said current detecting means. When the current in said inductive element is nearly zero, said control means turns off said fourth switching element and turns on said second and third switching elements. In this way, it is possible to keep each end of the inductive element at the same potential, to eliminate changes in the inductor current, and to reduce the undesirable electric power losses. Also, in this case, it is possible to prevent ringing caused by the inductive element and the parasitic capacitance, and to lower the noise level.
Also, in the present invention, the following scheme is preferred: said control means enters first, second and third operating states repeatedly; in said first operating state, said first and third switching elements are turned on, and said second and fourth switching elements are turned off, in said second operating state, said first and third switching elements are turned off, and said second and fourth switching elements are turned on; and, in said third operating state, said first and fourth switching element are turned off, and said second and third switching elements are turned on.
Also, in the present invention, the following scheme is preferred: said control means enters first, second and third operating states repeatedly; in said first operating state, said first and fourth switching elements are turned on, and said second and third switching elements are turned off; in said second operating state, said first and third switching elements are turned off, and said second and fourth switching elements are turned on; and, in said third operating state, said first and fourth switching element are turned off, and said second and third switching elements are turned on.
Also, in the present invention, the following scheme is preferred: said control means enters first, second and third operating states repeatedly; in said first operating state, said first and third switching elements are turned on, and said second and fourth switching elements are turned off; in said second operating state, said first and fourth switching elements are turned on, and said second and third switching elements are turned off; and, in said third operating state, said first and fourth switching element are turned off, and said second and third switching elements are turned on.
In addition, according to the present invention, it is preferred that said first, second, third and fourth switching elements be made up of MOS transistors, and that a body diode be formed between the source and drain of each of said MOS transistors.
In addition, according to the present invention, it is preferred that when current through said inductive element is nearly zero, said control means turns on either of said second and third switching elements.
In addition, according to the present invention, the following scheme is preferred: it has a driver for supplying a switching control signal to the gate of each of the MOS transistors that form said first and fourth switching elements; said driver has a diode and a capacitor connected in series between the terminal for power source voltage supply and one terminal of said inductive element, and a buffer which has its power supply terminal connected to the connection point between said diode and capacitor, its reference voltage terminal connected to one terminal of said inductive element, its input terminal for receiving the control signal from said controller input, and its output terminal supplying said switch control signal to the gate of the MOS transistor of the control object.
The present invention also provides a type of controller characterized by the following facts: the controller is used to control said first, second, third and fourth switching elements of said DCxe2x80x94DC converter, which has an inductive element, a first switching element connected between a first power source terminal and one terminal of said inductive element; a second switching element connected between a second power source element and said terminal of said inductive element, a third switching element connected between said second power source terminal and the other terminal of said inductive element, a fourth switching element connected between a voltage output terminal and said other terminal of said inductive element, and a capacitive element connected to said voltage output terminal; wherein the operation of the controller is such that when said first and fourth switching elements are off, said second and third switching elements are turned on.
According to the present invention, it is preferred that it have a current detecting circuit that detects the current flowing through said inductive element, and operates such that when the current flowing through said inductive element is zero, said second and third switching elements are turned on.
In addition, according to the present invention, it is preferred that when said current detecting circuit detects that the current flowing through said inductive element is zero, the controller turns said first and fourth switching elements off and turns said second and third switching elements on.